gh-125420: implement Sequence.index API on memoryview objects (#125446)

Objects/clinic/memoryobject.c.h

@@ -418,4 +418,50 @@ skip_optional_pos:
 exit:
     return return_value;
 }
-/*[clinic end generated code: output=0a93f08110630633 input=a9049054013a1b77]*/
+
+PyDoc_STRVAR(memoryview_index__doc__,
+"index($self, value, start=0, stop=sys.maxsize, /)\n"
+"--\n"
+"\n"
+"Return the index of the first occurrence of a value.\n"
+"\n"
+"Raises ValueError if the value is not present.");
+
+#define MEMORYVIEW_INDEX_METHODDEF    \
+    {"index", _PyCFunction_CAST(memoryview_index), METH_FASTCALL, memoryview_index__doc__},
+
+static PyObject *
+memoryview_index_impl(PyMemoryViewObject *self, PyObject *value,
+                      Py_ssize_t start, Py_ssize_t stop);
+
+static PyObject *
+memoryview_index(PyMemoryViewObject *self, PyObject *const *args, Py_ssize_t nargs)
+{
+    PyObject *return_value = NULL;
+    PyObject *value;
+    Py_ssize_t start = 0;
+    Py_ssize_t stop = PY_SSIZE_T_MAX;
+
+    if (!_PyArg_CheckPositional("index", nargs, 1, 3)) {
+        goto exit;
+    }
+    value = args[0];
+    if (nargs < 2) {
+        goto skip_optional;
+    }
+    if (!_PyEval_SliceIndexNotNone(args[1], &start)) {
+        goto exit;
+    }
+    if (nargs < 3) {
+        goto skip_optional;
+    }
+    if (!_PyEval_SliceIndexNotNone(args[2], &stop)) {
+        goto exit;
+    }
+skip_optional:
+    return_value = memoryview_index_impl(self, value, start, stop);
+
+exit:
+    return return_value;
+}
+/*[clinic end generated code: output=2742d371dba7314f input=a9049054013a1b77]*/

Objects/memoryobject.c

@@ -2748,6 +2748,92 @@ static PySequenceMethods memory_as_sequence = {
 };
 
 
+/**************************************************************************/
+/*                             Lookup                                     */
+/**************************************************************************/
+
+/*[clinic input]
+memoryview.index
+
+    value: object
+    start: slice_index(accept={int}) = 0
+    stop: slice_index(accept={int}, c_default="PY_SSIZE_T_MAX") = sys.maxsize
+    /
+
+Return the index of the first occurrence of a value.
+
+Raises ValueError if the value is not present.
+[clinic start generated code]*/
+
+static PyObject *
+memoryview_index_impl(PyMemoryViewObject *self, PyObject *value,
+                      Py_ssize_t start, Py_ssize_t stop)
+/*[clinic end generated code: output=e0185e3819e549df input=0697a0165bf90b5a]*/
+{
+    const Py_buffer *view = &self->view;
+    CHECK_RELEASED(self);
+
+    if (view->ndim == 0) {
+        PyErr_SetString(PyExc_TypeError, "invalid lookup on 0-dim memory");
+        return NULL;
+    }
+
+    if (view->ndim == 1) {
+        Py_ssize_t n = view->shape[0];
+
+        if (start < 0) {
+            start = Py_MAX(start + n, 0);
+        }
+
+        if (stop < 0) {
+            stop = Py_MAX(stop + n, 0);
+        }
+
+        stop = Py_MIN(stop, n);
+        assert(stop >= 0);
+        assert(stop <= n);
+
+        start = Py_MIN(start, stop);
+        assert(0 <= start);
+        assert(start <= stop);
+
+        PyObject *obj = _PyObject_CAST(self);
+        for (Py_ssize_t index = start; index < stop; index++) {
+            // Note: while memoryviews can be mutated during iterations
+            // when calling the == operator, their shape cannot. As such,
+            // it is safe to assume that the index remains valid for the
+            // entire loop.
+            assert(index < n);
+
+            PyObject *item = memory_item(obj, index);
+            if (item == NULL) {
+                return NULL;
+            }
+            if (item == value) {
+                Py_DECREF(item);
+                return PyLong_FromSsize_t(index);
+            }
+            int contained = PyObject_RichCompareBool(item, value, Py_EQ);
+            Py_DECREF(item);
+            if (contained > 0) {  // more likely than 'contained < 0'
+                return PyLong_FromSsize_t(index);
+            }
+            else if (contained < 0) {
+                return NULL;
+            }
+        }
+
+        PyErr_SetString(PyExc_ValueError, "memoryview.index(x): x not found");
+        return NULL;
+    }
+
+    PyErr_SetString(PyExc_NotImplementedError,
+                    "multi-dimensional lookup is not implemented");
+    return NULL;
+
+}
+
+
 /**************************************************************************/
 /*                             Comparisons                                */
 /**************************************************************************/
@@ -3284,6 +3370,7 @@ static PyMethodDef memory_methods[] = {
     MEMORYVIEW_CAST_METHODDEF
     MEMORYVIEW_TOREADONLY_METHODDEF
     MEMORYVIEW__FROM_FLAGS_METHODDEF
+    MEMORYVIEW_INDEX_METHODDEF
     {"__enter__",   memory_enter, METH_NOARGS, NULL},
     {"__exit__",    memory_exit, METH_VARARGS, memory_exit_doc},
     {"__class_getitem__", Py_GenericAlias, METH_O|METH_CLASS, PyDoc_STR("See PEP 585")},